This invention relates to a method of preparing a plastic substrate to improve the characteristics of its surface for the bonding thereto of a subsequently applied metal film, such as an electrolessly deposited metal film. This invention relates to both an improved laminate comprising a plastic substrate and metal film, as well as to the substrate itself, useful especially in the production of "additive" circuit boards for electrical and electronic equipment.
The method here disclosed is generally similar to that described in U.S. Pat. Nos. 3,620,933 and 3,666,549, in that initially a sacrificial metal foil is bonded by heat and pressure to a surface of the polymer substrate which is ultimately to be metal plated or otherwise metallized. The sacrificial metal foil is chemically stripped or dissolved from the surface of the substrate, after which the permanent metal film is deposited by known techniques. Application of the sacrificial foil to, and subsequent chemical stripping of it from, the plastic surface produces a microporous topography on the substrate surface that provides improved bonding characteristics for electrolessly plated metal film.
This invention is directed to the improvement in the foregoing procedure obtained by combining with the processing steps previously taught, a treatment of the interface formed by the plastic substrate and initial or sacrificial metal foil to provide thereat what is believed to be a molecular film, or at least microscopic amounts, of an organic silicon compound. The treatment to provide such silicon compound at the interface is accomplished before lamination of the foil to the plastic, and generally it is preferred to accomplish this by immersing the anodized metal foil, or otherwise coating it, in a solution of the organic silicon compound. The improvement obtained by this step is evidenced not only in greater bonding or peel strength between the substrate and final metal film, but more especially in greater retention of such bonding strength after exposure of the laminate to elevated temperature during soldering.
One of the main requirements of printed circuits in general, and additive circuits in particular, is that they must exhibit strong bonding of the metal coating to the plastic substrate. The industry has adopted a minimum requirement of approximately 8 pounds per linear inch for adhesion between the conductor metal and the plastic substrate. Along with this is the further important requirement in a satisfactory printed circuit that the metal-to-polymer bond be stable at elevated temperatures up to around 500.degree. to 550.degree. F. Indeed, printed circuit boards as mass produced today are subjected to soldering operations designed to permanently mount the various electronic components that constitute the electrical circuit. Quite frequently, such soldering operation involves partially dipping the circuit board in a bath of molten solder in order to effect soldering of all junctions in one step. This represents a substantial thermal shock to the laminate. It is imperative, therefore, that such soldering operations not weaken the metal-to-polymer bond below the industry specification of minimum bond strength.
It has been found during extensive experimentation that many occasions arise where printed circuit boards show excellent metal-to-polymer adhesion at room temperature, but that a dramatic decrease or deterioration results because of soldering or other high temperature processing operations.
In the prior art there has also been some problem of criticality in anodizing the sacrificial metal foil, and in the time, temperature and pressure conditions employed in laminating the anodized foil to the substrate, in order to achieve consistent results on a commercial basis in respect to peel strength in the finished laminate.
It is accordingly an objective of the present invention to provide a method of producing consistently higher peel strengths between the metal conductor film and plastic substrate, to be able to do this over a wider range of operating conditions in the preparation of the laminate and thus provide greater tolerance for variables which inherently and unavoidably arise under commercial production operations, and especially to materially improve the thermal shock resistance of the final laminate product.
As noted briefly above, it has now been found that applying to the anodized sacrificial metal foil, prior to lamination of it to the substrate, a "film" of a suitable silicon derivative, more especially an organic silicon derivative of the class comprising the amino alkanoxy substituted silanes, one can substantially improve the adhesion of the conductor metal to the substrate both before and after soldering. The silicon derivatives can be applied by both aqueous and non-aqueous solutions. The concentration of the silicon derivatives in solution can be quite small, indicating that no more than perhaps a monomolecular layer is retained on the sacrificial metal foil prior to lamination. Surprisingly, it appears that the silicon derivative in its mono layer form is so tenaciously held at the plastic-metal interface that its effect is not diminished or destroyed in the process of laminating the metal to the polymer substrate or in the subsequent chemical stripping step.
The mechanism by which the silicon derivative exerts its favorable reaction is not well understood. It is somehow thought perhaps to be incorporated during lamination into the polymer surface and to protect this from degradation during soldering operations. Another possible mechanism could involve a direct bond between the electrolessly deposited metal, such as copper or nickel, and the silicon. Such metal-silicon bond is perhaps more durable than the bond of the metal to the substrate surface directly. Thus, one could visualize the electrolessly deposited metal as bonding to the silicon, and the silicon then bonding to the polymer substrate, in a bridging type of arrangement. A still further possibility for the mechanism of improved bond between the deposited metal and plastic substrate may be postulated on the basis of improved wetting or flow that is caused by the thin film of silicon derivative retained on the surface of the sacrificial metal prior to laminating. Indeed, improved flow of the plastic substrate into the microscopic crevices or capilliaries of the anodized sacrificial metal foil during lamination will result in closer reproduction of the intricate topography of that anodized surface by the plastic surface, thereby providing a more intimately interlocking contact, after the sacrificial metal has been chemically stripped, between the substrate and a subsequently deposited metal film.
Silicon derivatives, and silanes in particular, have been widely used in industry to promote the physical properties of various "filled" polymers. Filled polymers are made by blending into the polymer during its molding operation particles of titanium dioxide, asbestos, sand and other solids. Silanes have been used to promote the wetting of the solid particles with the polymer during the molding operation, thereby avoiding de-wetting or separation of the plastic from the filler material during mechanical stress. There are numerous references in the literature relating to the use of silicon derivatives or reactive silanes in various interfacial applications. One excellent reference is entitled "Reactive Silanes as Adhesion Promoters to Hydrophilic Surfaces," by Edmund P. Plueddenann, published by Dow Corning Corporation, Midland, Mich. So far as is now known, however, there has been no previous suggestion for the use of such silane materials in a sort of "transfer" type of process mechanism such as that apparently involved in this invention, wherein the silane is provided prior to lamination at the interface of metal-plastic members of a laminate structure, where this metal member is subsequently stripped away to better condition the plastic surface for selective metallization by an additive process such as electroless metal plating. It is a surprising feature of this invention that the effect of the silane derivative on the plastic surface exerts itself even after the sacrificial metal has been stripped off chemically. In other words, it would seem that the plastic surface directly adjacent to the sacrificial metal film is somehow "permanently modified" by the silane, and its action remains effective to improve adherence of the final metal film additively deposited on the surface of the substrate.
A wide variety of organic silicon derivatives is available but apparently not all are useful in the practice of the invention herein disclosed. The best results are obtained by use of silane-type materials, and more particularly there is preferred for commercial practice of the invention a rather specific type of silane having the general formula: EQU R . Si(R.sub.1).sub.3
wherein R is a lower alkyl (up to 6 carbons) amino substituted radical, and R.sub.1 is a lower alkanoxy (up to 3 carbons) radical.